1. Field of the Invention
This application relates to integrated circuit devices more particularly to testing for jitter present in integrated circuit devices.
2. Description of the Related Art
Many applications, such as optical transmission systems, use high speed clocks to synchronize the flow of data. Those high speed clocks typically include some level of jitter, which can lead to bit errors in the data being transmitted and possible degradation of system performance. Jitter is the variation in clock output frequency or phase from a desired output frequency or phase and can occur for a number of reasons. Jitter may be caused by noise introduced into the system from any of a variety of sources. An important area of jitter management is in the transmit path of an optical/electrical interface where the outgoing light pulses typically have jitter within tight system requirements. For example, for an OC-48 Synchronous Optical Network (SONET) system, having a data rate of approximately 2.5 GHz, transmit jitter is specified for frequencies from 12 KHz to 20 MHz. Other frequency bands may also be of interest for other applications. For example, an OC-192 SONET system specifies requirements for jitter frequencies from 50 KHz to 80 MHz.
In order to ensure that jitter specifications are being met, it can be important to accurately determine the jitter or phase noise present in a system. For high speed integrated circuits, measuring jitter can require expensive high speed equipment that can be time consuming to use. In addition, the external equipment allows noise sources to be coupled into the integrated circuit during measurement that may adversely affect the jitter measurement.
It would be desirable to provide a technique for measuring jitter performance that is low cost, reasonably reliable, and less complex than current techniques and can be used widely in evaluating and screening integrated circuits for jitter performance.